Screwdriver bits of this type have thus far been manufactured from alloyed tool steels that usually contain carbon and alloying additions, such as silicon, manganese, chrome, molybdenum and vanadium, in fractions of less than 1%. After hardening and tempering, these steels have a hardness of approximately 60–64 HRC. When used with power screwdrivers, the tips of the screwdriver bits manufactured from tool steel suffer from relatively high wear because they are subjected to higher stress than those of manual screwdrivers. It is desirable to extend the service life of screwdriver bits used in commercial applications, particularly those used in the installation of screws on automated production lines.
The manufacture of cross-tip screwdriver bits from metal-powder mixtures, i.e., from hard metals, has been attempted (DE 92 11 907 U1, DE 42 41 005 A1 and DE 43 00 446 A1). Here, the screwdriver bit blanks were manufactured by means of injection molding where flux was added to the hard metal powder. The flux was extracted from the injection-molded blanks during a subsequent process, and the blanks were then sintered to the final shape and density at a high temperature. Although screwdriver bits of hard metal have greater hardness than those of high-speed steel, they are so brittle that they fracture at torques lower than those commonly encountered in practice.
The design of pressing tools for manufacturing screwdriver bits of hard metal is described in VDI-Zeitschrift No. 7–9 (1999), pp. 42–45. Here, the blanks are directly pressed from metal powder. The above-mentioned article reported that crack-free, dimensionally stable screwdriver bit blanks can be manufactured by employing the described design of the pressing tools and the filling process with the aid of the finite-element method. However, neither the actual load values of the screwdriver bits that are mass-produced with this method nor whether these bits can meet the values required in practice is known. Screwdriver bits of this type have not been introduced to the market.
In contrast to the one-piece screwdriver bits discussed thus far, a screwdriver for cross-recess screws which consists of a shaft of relatively soft steel and a tip section of extremely hard material is described in U.S. Pat. No. 3,393,722. The bottom surface of the hard-metal tip section contains a pin that engages into a hole on the end surface of the shaft. The two parts are connected together by means of welding. The disadvantage of this design is that the connection between the tip section and the shaft by means of the cylindrical projection does not allow the transmission of torques from the shaft to the tip section unless the two parts are welded or soldered together. The tip section preferably consists of hard metal (tungsten carbide). The cross-tip profile of the tip section is relatively long, e.g., as long as those manufactured by conventional manufacturing methods, in which the cross-tip profile is produced by machining the grooves. However, such a long cross-tip profile is disadvantageous for hard-metal tip sections because hard metal is more brittle than steel and the long profile is unable to withstand high torques. In addition, this long profile is disadvantageous with respect to the manufacture of the tip section described further below. Screwdrivers or screwdriver bits of this design have not been introduced to the market, although the corresponding application was submitted more than 30 years ago and the demand for wear-resistant screwdrivers or screwdriver bits continues to increase. This also applies to a tool disclosed in DE 70 44 913 U1 in which a tip section of a high-strength material is connected to a shaft section of a material of lesser quality. Blanks of the tip section are preformed by means of a powder-metallurgical method. However, neither of these two documents contains any indications regarding the manufacturing method, the shaping, or the dimensions. Consequently, it can be assumed that no manufacturing methods that provided satisfactory results were found for these designs.
FR 2 469 250 discloses a cross-tip for a screwdriver which is manufactured from metal powder by means of pressing and subsequent sintering. The cross-tip profile of the penetrating section rises from a plane that extends perpendicular to the longitudinal axis of the tip body without a readily recognizable transition in the form of a radius or chamfer. On its rear side, the tip body contains a prismatic incision for producing the connection with the corresponding end of the screwdriver (shaft), wherein said connection should be realized by means of brazing. The length of the cruciform lands should approximately correspond to half the length of the tip body. In this case, steel or hard metal powder is used as the starting material.
In such a design, it is disadvantageous that the cruciform lands make the transition into the base plane without a radius or chamfer. Such sharp and abrupt transitions cause stress concentrations, in particular, with hard materials such as hard metal, where said stress concentrations significantly reduce the load bearing ability at this location, particularly torsional loads.
Another disadvantage can be seen in the described fastening method.
A self-centering of the two parts to be connected is not achieved with a continuous transversely extending incision. This self-centering can only be achieved with an auxiliary device, e.g., a ring that is stationarily placed onto the connecting point and cannot shift, not even during brazing.
Based on this state of the art, the invention aims to manufacture screwdriver bits of hard metal in such a way that the torques required in practical applications can be transmitted in the region of the penetrating sections due to the superior hardness, and that a significantly higher wear-resistance or a significantly longer service life is achieved than with conventional designs of this type, where the invention should also allow an inexpensive manufacture of the screwdriver bits.